1. Field of the Invention
The present invention relates in general to sensor technology and more specifically to wafer temperature measurement and calibration for semiconductor device thermal fabrication processes.
2. Description of the Prior Art
The current state-of-the-art in Ultra Large Scale Integration (ULSI) allows mass production of integrated circuits (IC's) in silicon with millions of transistors on one single chip at an affordable cost. An example of a mass-produced IC at affordable cost is the INTEL Pentium.RTM. microprocessor which has over three million transistors. Currently, it is fabricated with 0.35 .mu.m technology and there are plans to further reduce line width. A technology called Rapid Thermal Processing (RTP) has become a key for achieving improvements in smaller transistor dimensions and reduced connection resistance. In order to allow smaller transistor dimensions (0. 5 .mu.m and below) without short channel effects, very shallow junctions are necessary. RTP is currently a key technology in obtaining shallow junctions through the combination of a shallow ion implantation of As and B with a consecutive RTP step of approximately 1100.degree. C. for 10 seconds.
An RTP system consists of a chamber lined with a highly reflective, low emissivity material, such as gold. At the top and bottom of the chamber, heat sources are positioned. These are typically tungsten-halogen lamps which radiate energy in a waveband from 0.5-3.5.mu.m. Between the lamps, a central, processing portion of the chamber is formed by an upper and lower quartz barrier. The quartz barriers are transparent to radiation from the lamps. A silicon wafer is placed between the quartz barriers and processed. The silicon wafer is the only infrared absorbing object in the cavity, so a large amount of energy is coupled to the wafer, while the other parts of the chamber remain at a much lower temperature.
There are three major thermal processes in RTP: radiative thermal transport in the chamber; thermal conduction inside the wafer; and convective cooling of the wafer. In the process of manufacturing semiconductor devices various reactions such as oxidation, annealing, and chemical or physical vapor depositions occur on the surface of material being processed. Wafer temperature is a very influential parameter in controlling the physical properties of the material surface structure or films. As such, control of wafer temperature and uniformity of wafer temperature is a key parameter for achieving process control and uniformity.
The RTP presents a challenging environment for temperature monitoring and control. The semiconductor wafer has insignificant thermal mass. High levels of radiant energy can be focused on the wafer. Thus, wafer temperature can fluctuate rapidly and must be monitored closely.
Additionally, temperatures are likely to be non-uniform across the wafer surface. In an RTP oven the primary heating mechanism is radiant and thus wafer hot-spots are more likely. The hot spots can result either from localized peaks in the radiant energy of the lamps, or from differences in the emissivity of the semiconductor wafer surface.
Typically, wafer temperature control systems employ either of two wafer temperature sensing methods, namely contact method or non-contact methods. There are several techniques of contact method temperature sensing. One common technique is the use of contact sensors which measure the temperature of a body that the wafer rests on such as a hot plate. However, in environments of a moderate or high vacuum, the temperature difference between the body being sensed(hot plate) and the wafer is large enough to preclude accurate wafer temperature measurement. Another common technique is the use of a sensor which directly contacts the surface of the wafer. The major source of error associated with this technique is that there is often greater heat transfer between the wafer and the structure holding the sensor in contact with the wafer than between the wafer and the sensor itself The attachment of a thermocouple to a wafer is another method of contact temperature measurement. The attachment of a thermocouple to a wafer historically has been achieved either by bonding the thermocouple to the surface of the wafer, or by imbedding it in the wafer. However, the impracticality of inserting thermocouples into production wafers limits the utility of thermocouple wafers in process control.
Of the non-contact methods of temperature measurement, radiation thermometry or pyrometry is the most widely utilized. Radiation pyrometry involves measuring long wavelength radiation from an object and making a temperature determination on the basis of that measurement. The technique has several disadvantages. Among these disadvantages is the reliance on a surface optical emissive properties which vary with temperature, doping level and film properties.
Temperature probes are available which have been calibrated to absolute temperature standards in an isothermal fluid bath. The sensors are deeply immersed in the bath so that conduction along the leads and the sensors structure is minimal and thus the measurement point or junction is independent from any external lead temperature. Unfortunately, the process environment in which these probes are utilized, i.e. an RTP oven is typically non-isothermal. Therefore most energy sensors have limited absolute temperature accuracy when measuring temperatures in a non-isothermal environment. The reason for this is that most sensors have a lead system or a support structure and thermal conduction along the leads or the support structures extracts heat or pumps heats into the measurement junction, thus altering the temperature measurement.
What is needed is temperature monitoring and control system suitable for temperature control in an RTP oven.